1. Field of the Invention
The present invention generally relates to headphone apparatus and, more particularly, is directed to a headphone apparatus for reducing the circumference noise from the outside.
2. Description of the Related Art
In recent years, a headphone apparatus has been developed which has a function of electrically shielding the circumference noise from the outside. In such a headphone apparatus, a microphone is provided at the rear side of each of a pair of miniature speaker units placed over the right and left ears, and an audio signal picked up by the microphone is inverted in its phase and fed back to the input side of the corresponding speaker unit which thereby reproduces the audio signal. According to the thus constituted headphone apparatus, the audio signal thus fed back in opposite phase and reproduced by the speaker unit cancels sound transmitted to the ears from the outside, so that a person is prevented from hearing the circumference noise of the outside.
In the thus constituted headphone apparatus for shielding the circumference noise, however, since the microphone for picking up the signal to be fed back is provided at the rear side of the speaker unit, the microphone may pick up noise other than sound transmitted to the ear, for example, the sound of wind caused by movement of an object such as a hand near the ear. In this case, the phase of the feedback signal is not completely opposite to that of the sound transmitted to the ear from the outside due to the above-mentioned noise picked up by the microphone, so that undesirable sound is reproduced from the speaker unit and transmitted to the ear disadvantageously.
In order to obviate this problem, there has been developed a closed type headphone apparatus called a pressure type headphone apparatus in which a microphone for picking up the feedback signal is provided at the front side of the speaker unit. In the thus constituted closed type headphone apparatus, the space between the ear and the speaker unit is shielded from the outside by means of an ear pad with a good sound insulation efficiency, and the microphone for picking up the feedback signal is disposed within the shielded space. Therefore, since the microphone does not pick up undesirable external noise, only the circumference noise transmitted to the ear from the outside is fed back to the speaker unit and so the external circumference noise can be shielded efficiently.
However, when the thus constituted closed type headphone apparatus is configured as a headphone apparatus for shielding the external circumference noise, there is then the problem that the headphone apparatus can not be used for a long period of time. That is, since the closed type headphone apparatus is used in a state that the space between the ear and the speaker unit is shielded from the outside by means of the ear pad, the ear pad is urged or pressed against the ear side with a relatively large force. As a result, the feeling of the attachment of the headphone apparatus is not good and so it is difficult to continuously use the headphone apparatus for a long period of time. Further, when the closed type headphone apparatus is constructed as the headphone apparatus for shielding the external circumference noise to thereby completely shield the circumference noise, words spoken by a person around the headphone apparatus can not be heard at all, which can be dangerous depending on where the headphone apparatus is used.
If the headphone apparatus for shielding the circumference noise is used in an airplane, the pilot cannot hear an engine sound during the flight. The use of the closed type headphone apparatus is not preferable since it is difficult to continuously use the closed type headphone apparatus for a long period of time such as for a flight of several hours due to the unfavorable feeling thereof. Further, since sound such as an alarming sound broadcasted in the airplane is shielded by the use of the headphone apparatus for shielding the circumference noise, upon occurrence of an emergency, a reaction to the emergency may be delayed due to the usage of the headphone apparatus, which is an important problem in safety.